Method for removal of sulfur dioxide

ABSTRACT

A method for removing sulfur dioxide from gas streams, comprising contacting the gas stream with the aqueous absorption liquid, transferring the liquid to another location, then mildly heating the liquid to recover the sulfur dioxide. This absorption liquid allows removal of 99.9% of the sulfur dioxide, the recovery of very pure sulfur dioxide, is resistant to upsets due to variations in the gas stream, and is cost effective.

BACKGROUND OF THE INVENTION

The United States is one of the largest producers of sulfur in theworld. Much of the sulfur is produced as a by-product from theprocessing of gases containing hydrogen sulfide (H₂ S) and/or sulfurdioxide (SO₂), including natural gas, gas from crude oil production, andby-product gases from petroleum refining. Typically, in these and otherplants constructed during the 1980's, the H₂ S present in the gas streamthat is fed into the process ("feed gas") is converted by the process toSO₂, which is then converted by a catalytic process to elemental sulfur.This is normally accomplished by a number of well known processes, suchas the Claus Sulfur process, the Wellman-Lord process, and the Stretfordprocess.

In the Claus process, one-third of the H₂ S in the feed gas is burnedwith stoichiometric amounts of air to produce SO₂. The remaining H₂ Sthen reacts with SO₂ in a series of catalytic converters to formelemental sulfur. The residual SO₂ contained in the feed gas aftertreatment ("tail gas") is normally vented to the atmosphere. As emissionrequirements have become more stringent, these plants have had to beretrofitted to meet Clean Air Act regulations of the 1990's. Thisretrofitting utilizes new technology which is either a modification ofthe original process or an add-on process. Due to the large volume ofby-product sulfur being produced, the price of sulfur has declinedsteadily in the last five years from nearly $100 per ton to the currentprice of about $40 per ton. Consequently, new technology must beincreasingly effective, in terms of SO₂ removal efficiency, capitalcosts, and operating costs.

High SO₂ emissions in the flue-gas of coal-fired power plants have alsonecessitated more effective measures to control and dispose of SO₂. Someplants have adopted technology based on absorbing the SO₂ in throwawaylime or limestone solutions or slurries. The extremely corrosive natureof the solutions and slurries of these processes have made theiroperating costs quite prohibitive. Furthermore, disposal of the solidwaste generated by these processes has caused increasing environmentalconcerns.

In the limestone slurry systems, the SO₂ is reacted with a lime orlimestone slurry. The resulting slurry is then disposed of byland-farming or other means. The SO₂ removal efficiency of these slurrysystems is increased by the addition of pH buffers.

The use of either pure or impure dibasic acids as a pH-bufferingadditive in limestone-slurry systems is disclosed in a number ofarticles, including: Chi, "Using Byproducts: A Case Study," ChemTech, p.308, May, 1990; Chang, et al., Effect of Organic Acid Additives on SO₂Absorption into CaO/CaCO₃ Slurries," American Institute of ChemicalEngineers Journal, Vol. 28, No. 2, p. 261, March, 1982 (the preferred pHrange according to this article is 4-6); Chang, et al., "Testing andCommercialization of Byproduct Dibasic Acids as Buffer Additives forLimestone Flue Gas Desulfurization Systems," Journal of the AirPollution and Control Association, Vol. 33, No. 10, p. 955, October,1983; and, Lee, et al., "Oxidative Degradation of Organic AcidConjugated with Sulfite Oxidation in Flue Gas Desulfurization: Products,Kinetics, and Mechanism," Environmental Science and Technology, Vol. 21,No. 3, p. 266-272, 1987. The use of various dibasic acids has been shownto improve absorption rates of the limestone slurries.

For those processes that absorb the sulfur gases into a liquid first,much of the prior art utilizes an organic solvent, which chemicallyreacts with the SO₂ and H₂ S. Examples of this type of process includeShell's "Sulferox Process", Hydrocarbon Processing's Gas ProcessHandbook, Gulf Publishing, 1992; and those disclosed in U.S. Pat. No.3,832,454; U.S. Pat. No. 3,928,548; and U.S. Pat. No. 4,069,302.

All these processes that produce sulfur from sulfur gases are energyintensive and present hazards because of the use of organic solvents.Furthermore, most of these are not cost efficient, nor do they generallyachieve essentially complete removal of the sulfur gases.

Some removal processes utilize solvent-based reactions, but thenregenerate SO₂ instead of sulfur. The SO₂ released from the solvent canbe dried, and liquefied and sold for its chemical value. The marketprice for liquid SO₂ has remained relatively steady at nearly $225 perton in the last five years, which is far more than the selling price ofsulfur. An example of a solvent-based SO₂ generating technology isdisclosed in U.S. Pat. No. 4,885,146. All solvent-based systems sufferfrom high expense and the dangers normally associated with use ofsolvents.

The produced liquid SO₂ has a variety of industrial applications. LiquidSO₂ has been known as a good solvent for the purification of lubricatingoils and for increasing oil viscosity and paraffinity. It has also beenused as a solubilizing agent of phosphates and dyes, as raw material toproduce sulfuric acid and sulfolane, as an excellent polymer solvent,and for sulfonation with SO₂. Liquid SO₂ is usually manufactured by thesulfur-burning process and through recovery from metallurgical sources.Its availability from such sources, however, is subject to fluctuationsin economic conditions and the state of labor relations in themetallurgy industry. Recovery of liquid SO₂ from waste product of acidgas removal processes can certainly be a supplemental source to helpstabilize the liquid SO₂ supply to the industry.

To avoid the problems associated with solvent-based processes, someprocesses are aqueous-based. Such absorption liquids contain variouscomponents, mostly to enhance the absorption abilities of the liquid.For instance, numerous studies have shown that SO₂ absorption isenhanced at certain pH ranges, though there, are disagreements in theliterature as to exactly what is the optimum pH range. Regardless, asSO₂ is absorbed, the pH of the solution tends to be more acidic.Therefore, a buffering agent must be added to keep the pH in the properrange. U.S. Pat. No. 4,965,062 is an example of an aqueous-based system,and discloses a method for reacting H₂ S to elemental sulfur. Thisaqueous-based process uses sulfite ions and an acetic acid - acid saltbuffering system. This system requires H₂ S recycle to properly work.

Various aqueous processes using a sodium citrate absorption solutionhave been in use since the early 1970's. Sodium citrate is a popularabsorption product because it tends to buffer the absorption solution tokeep it in the pH range of 3.5 to 5.5, where maximum absorption anddesorption of SO₂ can occur. The absorbed SO₂ can either be reacted toproduce elemental sulfur, or recovered unaltered. See InformationCirculars 7774, 8540, 8793, 8806, and 8819, Bureau of Mines, UnitedStates Department of the Interior. Other similar processes that reactthe SO₂ to elemental sulfur are disclosed in U.S. Pat. No. 4,048,293;U.S. Pat. No. 4,519,994; U.S. Pat. No. 3,983,225; and, U.S. Pat. No.4,450,145.

Some processes concentrate and recover the SO₂ unaltered, instead ofreacting it to elemental sulfur. The recovered SO₂ can then be used asfeed for another process or liquefied and sold, as discussed above. Thecitrate process discussed above can be used for absorption anddesorption of SO₂, as for example disclosed in U.S. Pat. No. 3,886,069.

Other similar aqueous processes are disclosed in the following;Bengtsson, "The Flakt-Boliden SO₂ Recovery Process", Chemistry inCanada, January, 1981; Aqueous Absorbents for Stack Gas Desulfurizationby Absorption/Stripping, Electric Power Research Institute, CS-3185,July 1983; "The Recovery of Sulfur from Smelter Gases, " Journal of theSociety of Chemical Industry, Vol. 56, p. 139, May, 1937; U.S. Pat. No.4,181,506; "Union Carbide Claims 99% Effectiveness for Flue GasScrubber", Vol. 89, No. 46, Oil and Gas Journal, Nov. 18, 1991; ElectricPower Research Institute Report, CS-3228, Final Report, October 1983;Erga, "A New Regenerable Process for the Recovery of SO₂ ", ChemicalEngineering Technology, Vol. 11, p. 402-407, 1988; Erga, "SO₂ Recoveryby Means of Adipic Acid Buffers", Industrial Chemical EngineeringFundamentals, Vol. 25, p. 692-695, 1986; Goar, "Today's Sulfur RecoveryProcesses", Hydrocarbon Processing, Vol. 47, No. 9, p. 249-252, 1968;Kumazawa, "Simultaneous Removal of NO and SO₂ by Absorption into AqueousMixed Solutions", American Institute of Chemical Engineers Journal, Vol.34, No. 7, pp. 1215-1220; Johnstone, et al., "Recovery of Sulfur Dioxidefrom Waste Gases", Industrial and Engineering Chemistry, p. 101-109,January, 1938.

All of the processes for SO₂ removal discussed above, whether aqueous ornon-aqueous, suffer from some related problems. First, most of theseprocesses cannot reduce the amount of SO₂ in the effluent gas stream tobelow 100 ppm. One key reason for this lack of efficiency is the lack ofsufficient solubility of SO₂ in the absorption liquid. Another reason isthe lack of efficient contacting between the SO₂ and the absorptionliquid. Considering current and near-future EPA regulations on emissionsof SO₂, reduction of the amount of SO₂ remaining in the stream is anecessity and reduction to near zero may soon be required.

Second, almost all of these systems are not cost effective and in factthey lose money. That is, they cost more to operate than is made byselling the recovered sulfur or SO₂. These systems are expensive tooperate because of high energy consumption, as most require elevatedtemperatures. Also, the systems that absorb and regenerate SO₂ tend torapidly build up byproducts in the absorption liquid and thesebyproducts must be removed to maintain the efficiency of the processes.Also, many of the components used in the absorption liquid arerelatively expensive, hazardous to use, or corrosive, and thus requiremore expensive handling equipment.

Third, many of these processes do not tolerate variations in theincoming feed stream very well, and are easily upset.

A more cost efficient SO₂ removal process developed to date is theAquaclaus process. In the Aquaclaus process, the SO₂ is absorbed in anaqueous solution containing phosphoric acid (H₃ PO₄) and sodiumcarbonate (Na₂ CO₃), with the active chemical species being sodiumphosphate. The absorbed SO₂ is reacted with H₂ S to generate elementalsulfur. The sulfur is separated from the absorption liquid, and theabsorption liquid is recycled for use again. For a basic discussion ofthis process, see Hayford, "Process Cleans Tail Gases," HydrocarbonProcessing, Vol. 52, No. 10, p. 95-96, 1973. One of the desirableaspects of this process is its resistance to upsets due to variations inthe feed stream, as well as its high efficiency of removal.

However, what is needed in the marketplace and what is embodied in thepresent invention is a SO₂ removal process that is profitable to operateand removes virtually all the SO₂ from the feed stream in one pass,regardless of variations in the feed stream.

SUMMARY OF THE INVENTION

A proposed solution to the problems identified above, is to use amodified Aquaclaus process that removes the SO₂ by contacting thetail-gas or power plant flue-gas with a lean physical solvent in anabsorber. The effluent from such a process will contain less than 100parts per million by volume (ppmv) SO₂, and can be vented to theatmosphere. Preferably the effluent will have no SO₂ remaining. Thesolvent, rich in SO₂ from the absorber, is then sent to a regeneratorwhere the absorbed SO₂ is released by mild heating. The lean solvent isthen recycled for reuse.

The invention comprises a new synergistic blend to create a superiorabsorption liquid. This mixture comprises a polyprotic inorganic acid,an inorganic base, and an additive that further increases the bufferingcapacity of the absorption liquid, thereby enhancing the solubility ofSO₂ in said absorption solution, while maintaining effective SO₂regeneration. Said polyprotic inorganic acid is preferably phosphoricacid (H₃ PO₄). Said inorganic base is comprised of one of the following:(1) carbonates of an alkali metal, such as sodium carbonate (Na₂ CO₃),and/or potassium carbonate (K₂ CO₃); (2) hydroxides of an alkali metal,such as sodium hydroxide (NaOH), and/or potassium hydroxide (KOH); or(3) a mixture of the proceeding two inorganic bases. This additive issoluble or at least miscible in aqueous solution. This additive iscomprised of one of the following: (1) a monoprotic organic acid, suchas acetic acid (CH₃ CO₂ H), formic acid (HCO₂ H), lactic acid (CH₃CH(OH)CO₂ H), or propionic acid (C₂ H₅ CO₂ H); (2) a polyprotic organicacid such as adipic acid (HO₂ C(CH₂)₄ CO₂ H), tartaric acid (HO₂CH(OH)CH(OH)CO₂ H), isophthalic acid (C₆ H₄ -1,3-(CO₂ H)₂), or citricacid (HO₂ CCH₂ C(OH)(CO₂ H)CH₂ CO₂ H); 3 an organic base buffer such asethylene glycol (HOCH₂ CH₂ OH); (4) a mixture of any of the proceedingthree organic additives; (5) a polyprotic inorganic acid such as boricacid (H₃ BO₃); or (6) a mixture of the preceeding four organic and/orinorganic additives. This liquid has superior absorption, desorption andbuffering properties, is relatively safe and inexpensive to use, isresistant to variations in flow rates, has physical characteristics thatmake the solution easy to work with, and can be adapted for use withmost current equipment and systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing bisulfite-sulfite distribution as a functionof pH.

DETAILED DESCRIPTION OF THE INVENTION

The proposed process is an improved physical absorption process, usingsynergistically blended buffered aqueous solvents, similar to thatdescribed in the Aquaclaus process developed by Stauffer ChemicalCompany, and disclosed in U.S. Pat. No. 4,181,506. The basic ingredientsof the inventive absorption liquid are dilute amount of phosphoric acid(H₃ PO₄) and sodium carbonate (Na₂ CO₃), which are chemically inert tothe constituents of the feed gas under the operating conditions. Asignificant and innovative concept of the process is the use of anadditive which acts synergistically as a buffering agent and is misciblein the aqueous solvent system. Prescence of such an additivedramatically enhance the solubility of SO₂. Additionally, the boilingpoint of the solvent is elevated, thus reducing solvent loss duringregeneration.

Solubility of SO₂ in the absorption liquid is one of the key parametersthat determines the ability and efficiency of any particular process forSO₂ removal. The invention utilizes a novel absorption liquid. The firstcomponent of the absorption liquid is a polyprotic acid, e.g., H₃ PO₄.Phosphoric acid is polyprotic, i.e., it has more than one OH group andtherefore several H⁺ (proton) groups for ionization. It acts as anexcellent buffer, having a higher capacity than common monoproticbuffers to resist changes to the pH of the solvent. Typical acidiccomponents of the gas, i.e., SO₂, are absorbed to form a physicalsolution, which can be regenerated by disruption using direct orindirect heating to release the dissolved SO₂.

The fundamental reaction mechanism of SO₂ absorption in aqueous phase isSO₂ hydrolysis. When SO₂ is dissolved in water, a portion of it ionizesaccording to the following reactions:

    SO.sub.2 (g)←→SO.sub.2 (aq)                    (1)

    SO.sub.2 (aq)←→HSO.sub.3.sup.- +H.sup.+        (2)

    HSO.sub.3.sup.- ←→SO.sub.3.sup.2- +H.sup.+     (3)

Equilibrium conditions for reactions (1) to (3), i.e., bisulfate(HSO₃⁻)-sulfite(SO₃ ²⁻) distribution, are defined by the curves in FIG. 1.However, due to its thermodynamic limitation, the formation of HSO₃ ⁻and SO₃ ²⁻ from SO₂ in water are suppressed by increasing acidity,driving the reactions to the left and thereby reducing the solubility ofSO₂ in the aqueous phase, and thereby the resulting reaction rate.

As shown in FIG. 1, the hydrolysis capacity of SO₂ in aqueous phase isbest where the pH is within the range of 2 to 7. The desirableproperties of a buffered solution for SO₂ removal includes: (1) amaximum buffer capacity near a pH range where both absorption anddesorption rates are effective; and, (2) an additional significantbuffer capacity contributed by the addition of a small amount of adifferent buffer which substantially increases SO₂ uptake. Using H₃ PO₄acid buffer in a dilute amount, the monohydrogen and tribasic forms ofthe phosphate buffer react with the hydrogen ions to keep the solutionpH in the proper range and thus achieve high SO₂ loading (i.e.,solubility):

    HPO.sub.4.sup.2- +H.sup.+ ←→H.sub.2 PO.sub.4.sup.-(4)

    H.sub.2 PO.sub.4.sup.- +H.sup.+ ←→H.sub.3 PO.sub.4(5)

Such absorption potential by the phosphate buffer has been shown inseveral U.S. patents, including U.S. Pat. Nos. 3,911,093, 4,450,145, and4,519,994. Our laboratory testing indicates that a range of molarity ofphosphoric acid may be used, preferably less than 2.0M, with 1.5M beingoptimal.

Small amounts of sulfate (SO₄ ²⁻) may also be formed in the presence ofoxygen according to the following reaction:

    HSO.sub.3.sup.- +1/2O.sub.2 →SO.sub.4.sup.2- +H.sup.+(6)

The sulfate radical is the primary free radical responsible for solventdegradation. Oxidation reaction (6), however, is very limited and hencedoes not have a significant effect on the performance of the process. Tofurther reduce this effect, an inorganic base such as Na₂ CO₃ is addedto the absorption liquid. This base reacts with the sulfate radicals tolimit solvent degradation. In fact, laboratory testing has shown thatuse of the disclosed absorption liquid greatly reduced the rate of buildup of sulfate and helps increase SO₂ uptake. The preferred ratio of Na⁺to PO₄ ²⁻ is from 1.5 to 2.

After a period of time, accumulated sulfate must be purged from thesystem. This is done by cooling a slipstream of lean solution toprecipitate sodium sulfate decahydrate (Na₂ SO₄ •10H₂ O), also known asGlauber's salt.

    Na.sub.2 CO.sub.3 +SO.sub.4.sup.2- +2H.sup.+ +9H.sub.2 O→CO.sub.2(g) +Na.sub.2 SO.sub.4 •10H.sub.2 O.sub.(s)             (7)

The sulfate crystals are easily formed and removed from the solution bya screen.

A third component in the absorption liquid is a synergistic additive.For example, dibasic organic adipic acid, (H₂ Ad, i.e., COOH(CH₂)₄COOH), is also a polyprotic acid buffer. Similar buffer reactions takeplace as follows:

    Ad.sup.2- +H.sup.+ ←→HAd.sup.-                 (8)

    HAd.sup.- +H.sup.+ ←→H.sub.2 Ad                (9)

Its maximum buffer capacity lies in the range of most effective SO₂hydrolysis. H₂ Ad was selected because of its solubility in water, lowvolatility, stability, low toxicity, low cost, availability, andpotentially lower degradation rate. Laboratory testing indicates that anadditional 1000 ppm of H2Ad is beneficial.

Interestingly, the use of a small amount of adipic acid blended with thephosphoric acid yields radically better absorption rates than eitherbuffer alone.

Another choice of additive is dibasic organic base buffer, such asethylene glycol. Its buffer reactions are identical to that of H₂ Ad. Itwas selected for its solubility in water, low volatility, availability,and anti-oxidation ability that is normally present in the commercialbrands. Testing indicates an additional 1000 ppm of this product is alsobeneficial.

Regeneration usually requires a solvent of low volatility. The inventiveabsorption liquid has a distinct advantage over prior art absorptionliquids, as its components are non-volatile. Desorption is accomplishedby either lowering the vapor pressure above the absorption liquid (aswith a vacuum), or by raising the temperature of the absorption liquid.

An additional benefit of the disclosed absorption liquid is that it hasa higher boiling point than other absorption liquids, and thereforethere occurs less loss of solvent during regeneration of the SO₂.Furthermore, with less heat being utilized to vaporize solvent, a higherpercentage of the heat applied to the SO₂ -rich liquid is used tovaporize the SO₂. As heat is a cost in most applications, the use of theinventive method results in decreased operating costs.

Prior to liquefaction of the SO₂ that is regenerated, it needs to bedehydrated to less than 1 ppmv water vapor. This is necessary to preventhydrate formation and corrosion from condensed water. For this purposealone, the regeneration process should be conducted at slightly morethan one atmosphere pressure, e.g., 40 KPa. Under these conditions,there is less azeotrope of water vapor, so the production of scale inthe dehydration equipment can be reduced without reducing theequipment's capacity. Furthermore, operating at slightly elevatedpressure will also reduce the loss of solvent during solventregeneration.

The entire disclosed process is well-integrated, self-balancing, and tosome extent self-sufficient, as the steam requirement for the strippingprocess can be met by the energy recovered from both the waste heatboiler and the feed gas adiabatic cooler. In addition, typical heatrequirements in the stripper are small for physical solvents compared tochemical solvents because with chemical solvents more energy is neededto break the stable chemical bonds between the acid gas component andthe solvent. Furthermore, the use of additives creates the potential forobtaining a substantial reduction in steam consumption which does notexist in the original Aquaclaus Process. Steam consumption can bereduced in the invention because, as SO₂ removal efficiency increases,the liquid circulation rate can be reduced.

Therefore, as long as the operating conditions such as the solventcirculation rate, temperature, solvent concentration and pH can bemaintained at a relatively constant level, this process is verytolerant. The proposed process is virtually insensitive to variation infeed rate and composition such that upsets and variations in upstreamprocessing have little effect on its performance. Small amounts ofimpurities, such as CO₂ and CS₂, can also be tolerated. The disclosedabsorbing solution has proven to be stable and resistant to poisoning,and no expensive catalyst is required.

Tests described by other similar research and in several U.S. patents inthe early eighties restated the advantages of the absorption-desorptionconcept as having relatively low capital investment requirements,operating simplicity and flexibility with high SO₂ removal efficiency,and the resulting emission of very clean vent gas (i.e., less than 100ppmv SO₂). Due to a high level of process integration, processeffluents, which normally require treatment to be in compliance withlocal environmental regulations, have either no or very minimal levelsof unacceptable products. As a result, equipment and operating costs areextremely competitive with conventional processes because of themoderate operating conditions. The following examples illustrate thepresent invention in more detail.

EXAMPLE 1

This example describes the general procedure used to generate the datain the later examples. A solution of 8.25% Na₂ CO₃ by weight (wt) wasprepared by dissolving 89.8790 g of Na₂ CO₃ into one liter of water. Onehundred milliliters (ml) of the absorption solution with a Na/PO₄ ratioof 1.57 and 1.0 molar H₃ PO₄ was prepared by mixing 7.42 ml of 85 wt %H₃ PO₄ solution and 92.58 ml of 8.25 wt % Na₂ CO₃ solution. Fifty ml ofthe absorption solution was placed in a standard gas bubbler. A pure SO₂gas stream was bubbled through the absorption solution in the gasbubbler and exited to the vent hood. The gas bubbler with the absorptionsolution was weighed before and periodically during the test until therewas no more weight gain. The result was then calculated to give themaximum SO₂ solubility in grammole of SO₂ per liter of absorptionsolution (gmol/L) . The following tables illustrate the effect ofdifferent H₃ PO₄ concentrations and Na/PO₄ ratios in the absorptionsolution at 21.5° C. using pure SO₂.

    ______________________________________                                        H.sub.3 PO.sub.4 (M) at Na/PO.sub.4 ratio of 1.57                                                 SO.sub.2 (gmoVL)                                          ______________________________________                                        0.135               1.480                                                     0.81                1.933                                                     1.0                 1.970                                                     1.5                 2.201                                                     3.0                 2.314                                                     ______________________________________                                        Na/PO.sub.4 ratio                                                                           1.0 MH.sub.3 PO.sub.4                                                                   1.5M H.sub.3 PO.sub.4                                 ______________________________________                                        0             1.302     1.186                                                 1.0           1.346     1.639                                                 1.57          1.970     2.201                                                 1.97          2.276     2.195                                                 2.67          2.398     2.407                                                 4.0           2.932     3.647                                                 ______________________________________                                    

EXAMPLE 2

Using the same procedure described in Example 1, an absorption solution(Na/PO₄ ratio of 1.57 and 1.0 molar H₃ PO₄) with 1000 ppmv H₂ Ad as anadditive for absorbing SO₂. The effect of this additive on SO₂solubility (gmol/L) at various SO₂ gas concentrations at 21.5° C. isillustrated in the following table:

    ______________________________________                                        SO.sub.2 inlet concentration (%)                                                            With no H.sub.2 Ad                                                                        With 1000 ppmv H.sub.2 Ad                           ______________________________________                                         3            0.115       0.184                                               100           1.028       1.416                                               ______________________________________                                    

EXAMPLE 3

A series of runs were made employing the method described in Example 1in which the temperature of the absorption liquid was varied in order tostudy the effect of this parameter on SO₂ solubility, using pure SO₂. A50 ml flask with 25 ml absorption solution submerged in a water bath tomaintain constant temperature was used for this experiment. The resultsfor an absorption solution with a Na/PO₄ ratio of 1.57 and using 1.0molar PO₄, are illustrated in the following table:

    ______________________________________                                        Temperature (C.)                                                                             SO.sub.2 (gmol/L)                                              ______________________________________                                        21.5'          1.537                                                          30             1.360                                                          40             0.970                                                          50             0.821                                                          60             0.365                                                          ______________________________________                                    

EXAMPLE 4

In this example, the conditions are identical to those used in Example3. The effect of temperature on SO₂ solubility in the presence of 1000ppmv H₂ Ad as an additive is illustrated in the following table:

    ______________________________________                                        Temperature (C.)                                                                             SO.sub.2 (gmol/L)                                              ______________________________________                                        21.5           1.628                                                          30             1.456                                                          40             1.134                                                          50             0.934                                                          60             0.886                                                          ______________________________________                                    

EXAMPLE 5

In this example, the conditions are identical to those used in Example2. The effect of different additive concentrations (the additive herebeing H₂ Ad) on SO₂ solubility using pure SO₂ at 21.5° C. is illustratedin the following table:

    ______________________________________                                        H.sub.2 Ad (M)                                                                             SO.sub.2 (gmol/L)                                                ______________________________________                                        0            1.029                                                            0.005        1.193                                                            0.01         1.412                                                            0.05         1.730                                                            0.1          2.008                                                            0.2          1.876                                                            ______________________________________                                    

EXAMPLE 6

In this example, the conditions are identical to those used in Example2. Eight different additives having functionality similar to H₂ Ad, wereused in the absorption solution with a Na/PO₄ ratio of 1.57 and using1.0 molar PO₄ ²⁻. The results at 21.5° C. using pure SO₂ are illustratedin the following table:

    ______________________________________                                        Additive, each 1000 ppmv                                                                        SO.sub.2 (gmol/L)                                           ______________________________________                                        Propionic Acid    2.070                                                       H.sub.2 Ad        1.973                                                       Tartaric Acid     1.842                                                       Isophthalic Acid  1.917                                                       Citric Acid       1.961                                                       Ethylene Glycol   1.958                                                       Boric Acid        1.901                                                       H.sub.2 Ad and Ethylene Glycol                                                                  2.223                                                       ______________________________________                                    

EXAMPLE 7

In this example, the reaction apparatus is identical to those used inExample 3. Absorption solutions with a base/PO₄ ratio of 1.57 and using1.0 molar Po₄ ²⁻, employing different inorganic bases were used forabsorbing SO₂. The results on SO₂ solubility with the presence of H₂ Adas an additive at 21.5° C. using pure SO₂ are illustrated in thefollowing table:

    ______________________________________                                        H.sub.2 Ad (ppmv)                                                                         Na.sub.2 CO.sub.3                                                                          K.sub.2 CO.sub.3                                                                      NaOH                                         ______________________________________                                          0         1.537        1.471   1.808                                        1000        1.628        1.833   1.697                                        ______________________________________                                    

EXAMPLE 8

This example illustrates the regeneration ability of the absorptionsolution, in that the dissolved SO₂ can be released from the absorptionsolution using mild heating. A stripping apparatus employing a heated500 ml flask with a water-cooled reflex condenser was used. Fifty ml ofSO₂ loaded absorption solution with a Na/PO₄ ratio of 1.57, and using1.0 molar PO₄ ²⁻ with different additives was placed in the flask andheated to its boiling point. The flask was weighed before and after thestripping test. The results are illustrated in the following table:

    ______________________________________                                        Additive, each 1000 ppmv                                                                        % SO.sub.2 released                                         ______________________________________                                        No Additive       95                                                          Propionic Acid    60                                                          H.sub.2 Ad        65                                                          Isophthalic Acid  60                                                          Citric Acid       37                                                          Ethylene Glycol   30                                                          Boric Acid        35                                                          ______________________________________                                    

EXAMPLE 9

This example demonstrates the solubility of the absorption solutioncompared to water at 40° C. The solution was prepared the same way asdescribed in Example 1, and this solution had a Na/PO₄ ratio of 1.97,PO₄ ²⁻ of 1.5 molarity. A conventional packed tower with bell saddleswas used as the contacting device.

    ______________________________________                                                Gas      Liquid    Inlet SO.sub.2                                                                         Outlet SO.sub.2                                   Rate     Rate      Concentration                                                                          Concentration                             Solvents                                                                              (ml/min) (ml/min)  (ppm)    (ppm)                                     ______________________________________                                        Absorption                                                                            57.51    41.92     40.56     55.28                                    Solution                                                                      Water   57.74    43.27     39.34    166.51                                    ______________________________________                                    

EXAMPLE 10

In this example, the conditions are identical to those used in Example2. A mixture of additives having similar function as H₂ Ad, were used inthe absorption solution with Na/PO₄ ratio of 1.57 and 1.0 molar PO₄ ²⁻.The results at 21.5° C. using pure SO₂ are illustrated in the followingtable:

    ______________________________________                                        Additives, each 1000 ppmv                                                                              SO.sub.2 (gmol/L)                                    ______________________________________                                        Citric Acid, Boric Acid  1.909                                                Ethylene Glycol, Boric Acid                                                                            2.018                                                Ethylene Glycol, Citric Acid                                                                           1.862                                                Ethylene Glycol, Citric Acid, Boric Acid                                                               1.983                                                H.sub.2 Ad, Boric Acid   2.086                                                H.sub.2 Ad, Citric Acid  1.892                                                H.sub.2 Ad, Citric Acid, Boric Acid                                                                    1.951                                                H.sub.2 Ad, Ethylene Glycol                                                                            2.017                                                H.sub.2 Ad, Ethylene Glycol, Boric Acid                                                                2.046                                                H.sub.2 Ad, Ethylene Glycol, Citric Acid                                                               1.919                                                H.sub.2 Ad, Ethylene Glycol, Citric Acid, Boric Acid                                                   2.122                                                ______________________________________                                    

It will be understood by those in the art that various modifications andchanges could be made to the method and absorption liquid describedabove, and these modifications and change would not depart from thespirit and scope of the invention.

We claim:
 1. A method of removing sulfur dioxide from a gas stream,comprising;contacting said sulfur dioxide through a contacting meanswith an absorption liquid at a first location, said absorption liquidabsorbing said sulfur dioxide, said absorption liquid comprising; water;a polyprotic inorganic acid, to act as a buffering agent, to buffer thepH of said absorption liquid within the range of 2 to 7; an inorganicbase, selected from the group consisting of alkali metal carbonates andalkali hydroxides to aid in the removal of unwanted side products, andto enhance sulfur dioxide solubility; and, an additive, said additivechosen to synergistically enhance the absorption and solubility of saidsulfur dioxide and assist in buffering the pH, while still maintainingeffective sulfur dioxide regeneration, said additive consistingessentially of a mixture of adipic acid and ethylene glycol;transferring said absorption liquid containing said sulfur dioxide to asecond location; and, desorbing said sulfur dioxide by altering thedifferences between the vapor pressures of said sulfur dioxide in saidliquid and the gas phase above the surface of said absorption liquid,such that said vapor pressure of said sulfur dioxide in said absorptionliquid is greater than said vapor pressure of said gas above saidsurface of said absorption liquid.
 2. The method of claim 1, whereinsaid polyprotic inorganic acid is phosphoric acid.
 3. The method ofclaim 2, wherein said phosphoric acid has a molarity less than or equalto 2.0M in said absorption liquid.
 4. The method of claim 3, whereinsaid phosphoric acid has a molarity of 1.0M to 1.5M in said absorptionliquid.
 5. The method of claim 1, wherein said inorganic base is sodiumcarbonate.
 6. The method of claim 5, wherein the concentration ratio ofsaid sodium to phosphate is less than 3:1 in absorption liquid.
 7. Themethod of claim 6, wherein said concentration ratio of sodium tophosphate is between 1.5:1 and 2:1 in said absorption liquid.
 8. Themethod of claim 1 wherein said adipic acid has molarity equal to or lessthan 0.2M in said absorption liquid.
 9. The method of claim 8, whereinsaid adipic acid constitutes 1000 ppmv in said absorption liquid. 10.The method of claim 1, wherein said alteration of vapor pressures isaccomplished by heating said absorption liquid by use of a reboiler orsteam stripping.
 11. The method of claim 1, wherein said desorbing isconducted at a pressure slightly in excess of 1 atmosphere.
 12. Themethod of claim 1, wherein said polyprotic inorganic acid is phosphoricacid, said phosphoric acid having a molarity of 1.0M to 1.5M in saidabsorption liquid, wherein said inorganic base is sodium carbonate,wherein the concentration ratio of sodium to phosphate is between 1.5:1and 2:1, wherein said additive is a mixture of adipic acid and ethyleneglycol, said adipic acid constituting about 1000 ppmv in said absorptionliquid, and wherein said altering of vapor pressures is accomplished byheating said absorption liquid to a temperature of between 200° F. and240° F., and at a pressure of greater than 1 atmosphere.
 13. Anabsorption liquid for use in removal of sulfur dioxide from gas streamsthat is capable of regeneration of the absorbed SO₂ by the use of heatand pressure, said absorption liquid comprising:water; a polyproticinorganic acid, to act as a buffering agent, to buffer the pH of saidabsorption liquid within the range of 2 to 7; an inorganic base,selected from the group consisting of: alkali metal carbonates andalkali hydroxides to aid in the removal of unwanted side products, andto enhance sulfur dioxide solubility; and an additive, said additivechosen to synergistically enhance the absorption and solubility of saidsulfur dioxide and assist in buffering the pH, while still maintainingeffective sulfur dioxide regeneration, said additive consistingessentially of a mixture of: adipic acid and ethylene glycol.
 14. Theabsorption liquid of claim 13, wherein said polyprotic inorganic acid isphosphoric acid.
 15. The absorption liquid of claim 14, wherein saidphosphoric acid has a molarity less than or equal to 2.0 in saidabsorption liquid.
 16. The absorption liquid of claim 15, wherein saidphosphoric acid has a molarity of 1.0M to 1.5M in said absorptionliquid.
 17. The absorption liquid of claim 14, wherein said inorganicbase is sodium carbonate.
 18. The absorption liquid of claim 17, whereinthe concentration ratio of said sodium to said phosphate is less than3:1 in said absorption liquid.
 19. The absorption liquid of claim 18,wherein the concentration ratio of said sodium to said phosphate isbetween 1.5:1 and 2:1 in said absorption liquid.
 20. The absorptionliquid of claim 13, wherein said adipic acid has molarity equal to orless than 0.2M in said absorption liquid.
 21. The absorption liquid ofclaim 20, wherein said adipic acid constitutes 1000 ppmv in saidabsorption liquid.
 22. The method of claim 13, wherein said polyproticinorganic acid is phosphoric acid, said phosphoric acid having amolarity of 1.0M to 1.5M in said absorption liquid, wherein saidinorganic base is sodium carbonate, the concentration ratio of saidsodium to said phosphate being between 1.5:1 and 2:1, and wherein saidadditive is a mixture of adipic acid, said adipic acid and ethyleneglycol constituting 1000 ppmv in said absorption liquid.
 23. Anabsorption liquid for use in removal of sulfur dioxide from gas streamsthat is capable of regeneration of the absorbed SO₂ by the use of heatpressure, said absorption liquid comprising:water a phosphoric acid,said phosphoric acid having a molarity of 1.0M to 1.5M in saidabsorption liquid; a sodium carbonate, the concentration ratio of saidsodium to said phosphate being between 1.5:1 and 2:1; and an additive,said additive consisting essentially of adipic acid and ethylene glycol.